Slurry coating superalloys with fecraiy coatings

ABSTRACT

A SLURRY COATING PARTICULARLY FOR THENICKLE-BASE AND COBALT-BASE ALLOYS, WHICH COMPRISES, BY WEIGHT, 20-30 PERCENT CHROMIUM, 8-12 PERCENT ALUMINUM, 10-16 PERCENT SILICON, 0.1-3 PERCENT YTTRIUM, SCANDIUM OR LANTHANUM, BALANCE IRON IS APPLIED TO A SUTABLE SUBSTRATE AND DIFFUSION HEAT TREATED TO PROVIDE OXIDATION RESISTANCE THERETO.

United States Patent US. Cl. 117-46 CA 3 Claims ABSTRACT OF THE DISCLOSURE A slurry coating, particularly for the nickel-base and cobalt-base alloys, which comprises, by weight, 20-30 percent chromium, 8-12 percent aluminum, 10-16 percent silicon, 0.1-3 percent yttrium, scandium or lanthanum, balance iron is applied to a suitable substrate and diffusion heat treated to provide oxidation resistance thereto.

The invention herein described was made in the course of or under a contract with the Department of the Air Force. 7

BACKGROUND OF THE INVENTION The present invention relates in general to high temperature, oxidation resistant coatings and processes for achieving such coatings, particularly those of the iron/ chromium/ aluminum/ yttrium type.

In the patent to Talboom, Jr. et a1. 3,542,530 of common assignee with the present invention, there is described an advanced coating for the nickel-base and cobalt-base superalloys. The coating contemplated therein comprises, by weight, about 20-50 percent chromium, 10-20 percent aluminum, 0.03-2 percent yttrium or the like, balance iron. Because of the particular nature of this coating, specifically its high melting point, reference is made to the now conventional method of its application to a suitable substrate by vapor deposition from a molten pool of coating material in a vacuum chamber.

The vacuum vapor deposition techniques are essentially line-of-sight processes insofar as coating deposition is concerned and are, accordingly, most readily adapted to the coating of relatively small components, such as turbine blades or vanes. When the coating is desired on large parts or in internal passages shielded from the molten pool, the vapor deposition method becomes much more unwieldy and, in some instances, completely impractical.

In the patent to Joseph 3,102,044, also of common assignee with the present invention, there is described a slurry process for forming a protective aluminide coating on certain alloy parts. Since according to the Joseph technique a suitable coating slurry is merely painted or sprayed on the surfaces to be protected, with a subsequent heat treatment forming the desired protective aluminide, size of the part is of no importance, as distinguished from the vapor deposition methods.

The Joseph technique effects the generation of the protective coating by the reaction of aluminum in the slurry to form aluminides of the substrate metal from which the basic oxidation resistance is attained. The coatings of the iron/chromium/aluminum/yttrium type (hereinafter called FeCrAlY) depend, not as in Joseph on a reaction with the substrate, but rather on the formation of the FeCrAlY alloy at the surface-and within a certain compositional range for the reasons set forth in the Talboom, Jr. et al. patent.

The slurry of Joseph is also characterized by an active coating ingredient, i.e., aluminum, having a melting point substantially lower than the usual substrate alloy to be coated. In the Joseph process the applied coating becomes liquid during diffusion, and through reaction and diffusion with the base alloy resolidifies as the oxidation resistant aluminide having a melting point above the diffusion temperature.

With coatings of the FeCrAlY type, including CoCrAlY,

the melting point is typically higher than that of the substrate alloy or is at least so high that melting cannot take place wihout irreparable damage to the substrate. Thus, one cannot merely substitute the basic FeCrAlY coating components for the aluminum of Joseph and expect to generate the desired coating. Furthermore, in the slurry coating of the present invention, the coating components react with and dissolve a limited part of the base alloy, a combining of the elements in the coating with elements in the substrate, including those which may be harmful to oxidation resistance, providing oxidation resistant species. Thus the alloy microstructure as well as the coating mechanism differ from the aluminized parts and processes.

The generation of coating layers in the cobalt/chromium/aluminum/yttrium and nickel/chromium/aluminium/yttrium systems face similar problems. And in all of the coatings of the FeCrAlY-type, a substantial interchangeability of the iron group metals is tolerated.

SUMMARY OF THE INVENTION The present invention contemplates the inclusion of about 10-16 weight percent of silicon together with the other ingredients comprising the FeCrAlY-type coating in a suitable dispersant, forming a slurry for application to surfaces to be protected which are thereafter diffusion heat treated to form an oxidation-resistant layer.

The active ingredients for the FeCrAlY coating slurry comprise, by Weight, 20-30 percent chromium, 8-12 percent aluminum, 10-16 percent silicon, 0.1-3 percent yttrium, scandium or lanthanum, balance substantially iron. A suitable slurry is formed by mixing about 50 volume percent of suitably milled powders of the above composition with a volatile liquid dispersant, such as nitrocellulose.

With the above slurry an oxidation-resistant coating can be generated on the nickel-base superalloys, after the slurry has dried, by diffusion heat treatments at 2100 F. and 2225 F., but in any event below that temperature at which irreparable damage to the substrate occurs.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the coatings of the FeCrAlY-type, chromium provides hot corrosion resistance; aluminum furnishes oxidation resistance through the generation upon exposure to an oxidizing environment of the protective aluminum oxide; and yttrium, scandium and lanthanum promote adherence of the oxide reducing the spallation of the protective oxide. In the present slurry coating, silicon lowers the melting point of the coating powders and, in addition, reacts to tie up the molybdenum, when present in the substrate metal, as an oxidation resistant nickel/silicon/ molybdenum or nickel/silicon/aluminum/molybdenum compound. Pure molybdenum or a very high molybdenum phase is typically not oxidation-resistant.

The process hereinafter described in detail has been utilized to increase the oxidation, sulfidation, erosion and thermal shock resistance of the nickel-base superalloys. A nickel-base alloy of the composition, by weight, comprising 17.5-18.5 percent molybdenum, 7.75-8.25 percent aluminum, 0-0.05 percent carbon, balance nickel was thoroughly cleaned and degreased. The surfaces to be protected were then sprayed with a milled slurry of 50 volume percent 400 mesh powders of the composition comprising, by weight, 24-26 percent chromium, -105 percent aluminum, 14.5-15.5 percent silicon, 0.8-1.2 percent yttrium, balance substantially iron; and 50 volume percent nitrocellulose base lacquer. Drying was accomplished in air in 4 hours, providing a green coating.

The coated parts were heated to 2100 F. at a rate of 30-40 F. per minute and held at temperature for 10 minutes, then heated to 2225 F. and held for 5 minutes, in the absence of air, typically in a vacuum or hydrogen atmosphere. In general, coating thicknesses of 0.004- 0.007 inch after diffusion have been found satisfactory.

It is, of course, inherent in this coating system that softening occurs at a lower temperature than that of the basic FeCrAlY system. Thus, its utility is confined to those applications where exposure is confined to temperatures about 200 F. below the acceptable exposure temperature of the unmodified coating. The utility of the system has been established in a variety of applications, however, such as jet engine afterburner seal flaps.

The invention in its broader aspects is not limited to the specific steps, process and compositions shown and described but departures may be made therefrom within the scope of the appended claims without departure from the principles of the invention and without sacrificing its chief advantages.

What we claim is:

1. In the coating of the nickel-base and cobalt-base alloys with a composition of the FeCrAlY-type for high temperature oxidation-resistance, the improvement which comprises:

forming a mixture of prealloyed powder-s consisting essentially of said composition and -16 weight percent silicon;

dispersing said mixture in a compatible volatizable liquid to form a slurry;

applying the slurry to the alloy surface to be protected;

drying the slurry on the alloy to form a green coatand diffusion heat treating the green coating in the absence of air at a temperature above about 2100 F. to form the desired oxidation-resistant coating.

2. The method of coating a nickel-base or cobaltbase alloy to impart oxidation-resistance thereto which comprises:

forming a slurry comprising a powder mixture consisting essentially of, by weight, 20-30 percent chromium, 8-12 percent aluminum, 10-16 percent silicon, 0.1-3 percent yttrium, lanthanum or scandium, balance substantially iron, together with suificient volatizable liquid dispersant to provide the desired viscosity for application;

applying the slurry to the alloy to be protected;

drying the slurry on the alloy to form a green coating;

and ditfusion heat treating the coated part to form the desired oxidation-resistant coating.

3. The method of coating a nickel-base or cobalt-base alloy to impart oxidation resistance thereto which comprises:

forming a slurry comprising a powder mixture consisting essentially of, by weight, 24-26 percent chromium, -105 percent aluminum, 14.5-15.5 percent silicon, 0.8-1.2 percent yttrium, balance iron, to-

gether with a nitrocellulose dispersant sufiicient in quantity to provide the desired slurry viscosity; applying the slurry to the surface to be protected; drying the slurry on the alloy;

and heating the coated part at about 2100 F. to effect diffusion and form the desired oxidation-resistant coating.

References Cited UNITED STATES PATENTS 3,542,530 11/1970 Talboom, Jr., et al. 29194 X 3,477,831 11/1969 Talboom, Jr., et al.

117131 X 2,878,554 3/1959 Long 117-131 X 3,000,755 9/1961 Hanink et a1 117131 X 3,102,044 8/1963 Joseph 117131 X 3,300,854 1/1967 Jackson et al 117-131 X 3,447,912 6/1969 Ortner et al. 117--131 X 3,540,863 11/1970 Priceman et al. 117-131 X ALFRED L. LEAVITT, Primary Examiner J. R. BATTEN, JR., Assistant Examiner US. Cl. X.R. 

